Bug 1041124 - [Messages][Refactoring] Make use of EventDispatcher in MessageManager. r=julien

If you're not against Threads.js->MessageManager.js dependency I'll defintely try :) Thinking more about it, registering thread isn't responsibility of the any particular view/panel since Threads is global singleton that can be used by any view/panel. So yes, some other "Object" should register it (Threads, MessageManager or someone else, but not view/panel as done in this patch currently)